The present invention relates to aqueous dispersions containing cationic resins and capped polyisocyanate curing agents and to their use in electrodeposition processes.
The application of a coating by electrodeposition involves depositing a film-forming composition to an electrically conductive substrate under the influence of an applied electrical potential. Electrodeposition has gained prominence in the coatings industry because in comparison with non-electrophoretic coating methods, electrodeposition provides higher paint utilization, outstanding corrosion resistance, and low environmental contamination. Early attempts at commercial electrodeposition processes used anionic electrodeposition where the workpiece being coated served as the anode. However, in 1972 cationic electrodeposition was introduced commercially. Since that time cationic electrodeposition has become increasingly popular and today is the most prevalent method of electrodeposition. Throughout the world, the primer coat of choice for corrosion protection of motor vehicles is cationic electrodeposition.
Many cationic electrodeposition compositions used today are based on active hydrogen-containing resins derived from a polyepoxide and a capped aromatic or aliphatic polyisocyanate curing agent.
Typically, an aromatic polyisocyanate curing agent may be capped with an aliphatic alcohol including lower aliphatic alcohols such as methanol, ethanol, and n-butanol, or cycloaliphatic alcohols such as cyclohexanol. Glycol ethers are also conventionally used as capping agents. Such glycol ethers include ethylene glycol butyl ether, diethylene glycol butyl ether, ethylene glycol methyl ether and propylene glycol methyl ether. These conventional capping agents require cure temperatures in excess of 360.degree. F. (182.degree. C.) unless catalysts are used. An aromatic or aliphatic polyisocyanate curing agent may also be capped with phenolic capping agents, wherein the phenolic hydroxyl group reacts with the isocyanate group in the polyisocyanate. Such capping agents deblock and allow for cure at lower temperatures but are known to be chemically unstable in electrodepositable compositions.
To reduce energy costs and to ensure sufficient cure over more massive components such as large parts, metal catalysts are usually included in conventional cationic electrodepositable compositions. Organotin compounds such as dibutyltin oxide, lead salts such as lead silicate, and bismuth salts are examples of such catalysts. In the presence of these catalysts, cure temperatures as low as 340.degree. F. (171.degree. C.) can be achieved with aromatic polyisocyanates. For alcohol blocked aliphatic polyisocyanate curing agents cure temperatures of 380.degree. F. (193.degree. C.) can be achieved. However, catalysts most useful in cationic electrodepositable compositions are either expensive or environmentally undesirable due to their appearance in electrocoat ultrafiltrate waste streams.
Also, the number of effective catalysts available and their ability to reduce cure temperatures below 340.degree. F. (171.degree. C.) for aromatic isocyanates [380.degree. F. (193.degree. C.) for aliphatic isocyanates] while maintaining performance properties such as corrosion resistance is severely limited. Of the known cationic electrodepositable compositions, only those containing lead have exhibited high corrosion resistance over substrates such as bare steel, and this effect is not achievable at temperatures below 340.degree. F. (171.degree. C.) without losing other performance properties, even when higher levels of lead or auxiliary catalysts are added.
Another common approach to producing capped aromatic polyisocyanate curing agents which cure at temperatures below 360.degree. F. (182.degree. C.) is to replace the aliphatic alcohol with a phenol or phenol derivative such as cresol. While these compositions cure at temperatures below 360.degree. F. (182.degree. C.), they exhibit poor chemical stability in electrocoat compositions and can also contaminate electrocoat ultrafiltrate.
Thus, there exists a need for cationic electrodepositable compositions with good stability which rely on minimal levels of metal catalysts that produce high performance, corrosion resistant coating when baked at temperatures below 340.degree. F. (171.degree. C.) for blocked polyaromatic isocyanates and 380.degree. F. (193.degree. C.) for blocked aliphatic polyisocyanates.